The present invention relates to ion transfer components for use in mass spectrometry.
A mass spectrometer analyzes mass-to-charge ratios of ions, and typically includes an ion source, ion optics, one or more mass analyzers, and one or more detectors. In the ion source, particles are ionized so that they can be influenced and manipulated via electrostatics. The ions may be transported through different vacuum stages using ion transfer optics, such as ion guides, to one or more mass analyzers, which separate the ions based on their mass-to-charge ratio. The separated ions are then detected by one or more detectors, which provide the data used to construct a mass spectrum of the sample.
Ion sources that operate at or near atmospheric pressure (Atmospheric Pressure Ionization or API sources) are useful for producing ions from non-volatile, higher molecular weight, molecules. The most prevalent use of API techniques in mass spectrometry (MS) is for ionization of molecules chromatographically separated using high pressure liquid chromatography techniques (LC-MS). The ability of API-MS techniques to handle a variety of LC flow rates with high analytical sensitivity, precision, and linearity, has made API-MS indispensable in the modern analytical laboratory.
There are two common types of API: atmospheric pressure chemical ionization (APCI) and electro-spray ionization (ESI). In APCI, a heated nebulizer is used to convert droplets of sample solution into the gaseous phase. A corona discharge electrode that is adjacent to the outlet of the nebulizer then ionizes the solvent and the sample molecules. In ESI, eluent containing the sample molecules of interest is passed through a small capillary. A strong potential, typically 1–5 KV, is maintained between the capillary and an adjacent surface such as the mass analyzer. As a result, the liquid disperses into fine ionized droplets when it emerges from the capillary. These droplets are heated, evaporating the solvent and making the droplets small and unstable, whereupon gaseous sample ions are formed.
Ions produced by API sources are typically transferred into a chamber that is at an intermediate pressure, e.g., lower than atmospheric pressure, and then directed into a chamber that is at low pressure, i.e., lower than the intermediate pressure chamber, via the use of ion optics. The ions enter the low-pressure chamber through an orifice, which is very small so that the low-pressure chamber can be maintained at low pressure. Ion transfer components such as electrostatic lenses with holes in them, multipole ion guides or skimmers are used to efficiently transfer ions from one region to another, typically across a pressure differential, for example from the intermediate pressure chamber to the low-pressure chamber.
Some ion transfer components, for example skimmers, reduce the undesirable effects of boundary conditions on the flow of ions and the resulting signal that is detected. For example, a conical shaped skimmer can reduce the effects of shock waves on the flow of ions. Other shapes and configurations are possible, but ion transfer components operating in the intermediate pressure regime typically have a sharp edge defining the orifice through which ions pass. Special configurations of systems that employ API and ion transfer components may further improve the quality of the resulting spectrum. For example, systems can be configured to use an off-axis alignment of the capillary tube and the skimmer, as described in U.S. Pat. No. 5,171,990, or to have various arrangements of trajectories, as described in U.S. Pat. No. 5,756,994.
Ion transfer components made of relatively inert, non-reactive materials such as stainless steel can reduce the accumulation of chemical deposits resulting from reaction of the ion transfer component with certain constituents of the gas. Such chemical deposits affect the surface flow dynamics of the ions and can compromise the performance of the skimmer. Ion transfer components can be made of glass, but they are subject to breakage when removed for cleaning or servicing. As described in U.S. Pat. No. 6,608,318 and U.S. Pat. Pub. 2003/0146378, inorganic conductive nitride compounds such as titanium nitride have been used to coat surfaces in the chamber and on the skimmer in order to reduce the accumulation of chemical deposits.